Various principles and methods are known in the field of electronic or electro-optical rangefinding. One approach consists of emitting pulsed electromagnetic radiation, such as e.g. laser light, to a target to be measured, and subsequently receiving an echo from this target as back-scattering object, with the distance to the target to be measured being determined on the basis of the time-of-flight of the pulse. Such pulse time-of-flight measuring devices have in the meantime prevailed as standard solutions in many fields.
However, current rangefinders equipped with a laser light source, for example from a laser scanner with a high measurement accuracy, in particular with low distance noise, often exhibit artifacts such as intensity noise within the data point clouds generated thereby, and indicate wavy, bumpy surfaces therein instead of actually present smooth, flat surfaces sampled and to be imaged. The occurrence of intensity noise of laser light emerges in a known fashion, particularly when targeting rough surfaces with laser light. The backscattered light here has grainy granulation in the brightness.
The grainy interference phenomena are denoted “speckled patterns” of scattered light or simply “speckles”, which phenomena can be observed when illuminating optically rough object surfaces with sufficient coherence in both time and space for causing this phenomenon. The unevenness of the light-scattering surfaces causing this has dimensions here of an order of magnitude of between the wavelength of the laser light and a few 10 μm.
Speckles are generated when highly coherent light radiation is incident on inhomogeneous surfaces; in the current case onto objects with a rough surface; the light is subsequently transmitted or reflected and propagates in the direction of the detector. The scattered light exhibits the aforementioned granulation. The scatted radiation has an irregular field and intensity distribution and an approximately cigar-like shape in the propagation direction (regions with high energy density). Then, a granular intensity distribution is observable on the reception lens of the distance sensor. The received power, and hence the reception signal, varies irregularly when the scanner with the laser beam is moved over the object to be measured. This effect provides an unnatural brightness reproduction of the object. Moreover, the distance measurement values are noisy.
The term “speckle” is used both for an individual light spot and for the whole interference pattern. Depending on the employed imaging system, a distinction is made between “subjective” and “objective” speckles: if the speckles are imaged directly onto a screen or a camera without the aid of a lens element or other optical devices, this is referred to as “objective” speckle.
The speckle distribution or the change therein can easily be observed in the case of “objective” speckles by means of a paper sheet as a projection surface in the surroundings of the surface illuminated by the laser. The mean dimensions of the speckles in these interference patterns are primarily determined by the wavelength of the coherent excitation light, the diameter of the excitation light beam or, equivalently thereto, the size of the illuminated area and the associated geometry.
By contrast, imaging the interference pattern with the aid of an optical system—this includes the human eye—is involved in the case of “subjective” speckles. If the light pattern generated by a scattering object is imaged by means of an optical system, the speckle pattern in the image is referred to as “subjective” and the mean speckle dimension then is dependent on the optical parameters of the imaging system, such as e.g. the focal length f and the pupil diameter de.
If the light source has a plurality of modes M, for example in a manner like pulsed Fabry-Perot (FP) laser diodes, M on average independent speckle fields are created. These M fields superpose incoherently, the intensities sum and the variation of the reception signal strength when scanning the laser beam over the target object is reduced by a factor of the square root of M. In the case of a spectrally broadband light source, such as e.g. a superluminescent diode (SLD), this smoothing effect is amplified since the spectrum is broader than in the case of conventional multimode laser diodes and, moreover, the spectrum of an SLD does not have gaps.
The typical dimension of the speckles at the point of the reception pupil, but also at the field stop in the case of the reception diode, can be calculated. The mean speckle diameter is approximately:
      d    speckle    :=                    π        ·        λ        ·        Dist                    d        spot              .  
Here, “Dist” means the distance between the illuminated surface and the reception pupil and “dspot” means the beam diameter on the illuminated surface.
In the case of close targets, the speckles tend to have fine granulation; this increases with increasing distance. If the measurement light of the laser is focused to infinity, it is possible to observe that the speckle dimension no longer increases anymore after a distance of a few 10 m and it assumes approximately the extent of the transmission beam in the transmission pupil.dspeckle=dTX 
In order to be able to describe speckles quantitatively, it is necessary to consider the electromagnetic field of the light scattered back from a rough surface in more detail. Here, the electromagnetic field varies in amplitude and phase. Mathematically, such phasors are described in complex form as a combination of a real and an imaginary field component, wherein, in the simplest case, both can be assumed as distributed in accordance with a Gaussian function by way of the scattering process. The associated distributions of the magnitude of the field amplitude and the phase are Rayleigh distributions or “equal” distributions. The distribution density function of the intensity of scattered, coherent light at one point of the reception aperture is negatively exponential:
      p    ⁡          (      I      )        =            1      Im        ·          exp      ⁡              (                              -            I                    Im                )            
Here, “I” means intensity measured at a point and “Im” means the mean value of the intensity of the radiation field. The contrast CON of such monochromatic radiation measured with a punctiform detector is CON=1.
There are a number of conventions for defining the speckle contrast. A frequently employed one is the following, which shows the relationship with the signal-to-noise ratio:
            Contrast      ⁢                          ⁢      of      ⁢                          ⁢      the      ⁢                          ⁢              intensity        :        CON              :=                            σ          ⁢                                          ⁢          I                Im            =              1                  S          ⁢                                          ⁢          N          ⁢                                          ⁢          R                                Reduction      ⁢                          ⁢      of      ⁢                          ⁢      the      ⁢                          ⁢              contrast        :        CON              =          1              M            
In the case of monochromatic laser light, like in the case of a DFB laser diode, M=1. Therefore, the contrast CON is 100% and the intensity measured at various points scatters by 100%. This value of the normalized brightness scattering is easy to derive by means of the associated statistical distribution density of the intensity (negatively exponential distribution).
In the case of a light source with a plurality M of modes, the contrast, and hence the intensity noise, decrease proportionally to the square root of M.
In the case of such a light source with a plurality of modes M, for example in a manner like pulsed Fabry-Perot laser diodes, M on average independent speckle fields are created. These superpose mainly incoherently, the intensities sum and the variation in the reception signal strength when scanning the laser beam over the target object reduces with root M.
The following equation specifies the distribution density of the intensity at a measurement point for a light source with M modes. This equation also applies to a spatial averaging process, as occurs in the case of a large reception aperture with M lateral correlation cells, i.e. there are M speckles in the reception aperture.
            Normalized      ⁢                          ⁢      intensity      ⁢                          ⁢              :            ⁢                          ⁢              I        k              :=                            k          K                ⁢                                  ⁢        I            =              [                  0          ⁢                                          ⁢          …          ⁢                                          ⁢          1                ]                        Mean      ⁢                          ⁢      intensity      ⁢                          ⁢              :            ⁢                          ⁢      Im        :=    0.5              Distribution      ⁢                          ⁢      density      ⁢                          ⁢      as      ⁢                          ⁢      a      ⁢                          ⁢      function      ⁢                          ⁢      of      ⁢                          ⁢      the      ⁢                          ⁢      intensity      ⁢                          ⁢      I      ⁢                          ⁢      and      ⁢                          ⁢      the        ⁢                      number    ⁢                  ⁢    M    ⁢                  ⁢    of    ⁢                  ⁢    modes    ⁢                  ⁢    or    ⁢                  ⁢    discrete    ⁢                  ⁢    laser    ⁢                  ⁢    wavelengths    ⁢                  ⁢          :                  p      ⁡              (                  I          ,          M                )              :=                            (                      M            Im                    )                M            ·                        I                      M            -            1                                    Γ          ⁡                      (            M            )                              ·              exp        ⁡                  (                                    -              M                        ·                          I              Im                                )                    
Here, Γ(M) denotes the gamma function.
Here, the depth roughness of the surface of the target object likewise plays a role. If the surface only has little roughness or if the transverse condition of the surface is too uniform (transverse correlation length), then the phases of the back-scattered partial waves of the electromagnetic field do not decorrelate sufficiently and the speckles indicate a residual brightness variation which lies between the exponential distribution and a Rice distribution with well-smoothed speckles.
The transverse dimension of the laser point on the target object also influences the quality of the measured point clouds. The observed dimension of the brightness spots, but also the regions of the distance errors (<0.5 mm), even if these are only small, when measuring homogeneous surfaces also have a relationship with the dimension of the laser point on the scanned object. In the case of an object scan, the laser beam is scanned over the surface, with the speckle pattern changing on the detector of the distance measurement sensor. The speckle pattern appears to change continuously but fluidly. The form of the pattern is decorrelated as soon as the laser measurement spot on the object has moved by one beam cross-section. Since a systematic distance measurement deviation is also linked to the speckle distribution, the measured surface exhibits a wavy distance deviation (bumpy surfaces).
In accordance with the prior art, distance measurement sensors for geodetic or industrial surveying instruments are generally equipped with a laser as light source. The following are typically used as lasers:                Laser diodes and solid-state lasers, in each case embodied with single mode or multi-mode spectra, wherein the multi-mode spectra typically have a width of approximately 1.5 nm;        So-called “seeded fiber amplifiers” (i.e. light fiber amplifiers amplifying the light from an excitation light source);        Fiber lasers;        “High radiance LEDs” (light-emitting high power diodes).        
Apart from the LEDs, all of the aforementioned light sources are connected with the disadvantage of a pronounced granular intensity distribution of the light scattered back from naturally scattering or reflecting surfaces. “High radiance LEDs” have the disadvantage of a low beam density and are therefore not used for measurements on diffusely scattering targets. The modulation speed of LEDs is also limited. The shortest signal increase times are of the order of a few nanoseconds. As a result of a too low modulation bandwidth, such diodes are no longer used for signal-sensitive rangefinders with a high accuracy.
Since speckles are a special phenomenon of spatially and time-coherent illumination, the speckle contrast and speckle influence can be reduced by various measures. By way of example, the following techniques are known for reducing speckles:                moving the transmission light spot on the target object;        moving the reception lens radially;        moving or vibrating a diffuser in the transmission beam across the beam direction;        using a diffuser with a small scattering angle;        using two diffusers in the transmission channel with the counter-directed movement thereof across the laser beam;        using “polarization diversity”, i.e. simultaneous emission of light with various polarization states.        
Since the light spot on the target object should be as small as possible, a reduction of the spatial coherence is generally not possible provided the object is not arranged very close and the transmission optical unit is focusable in such a way that the light spot maintains a sufficiently small extent on the target object. (The phrase “sufficiently small extent” in this case should be understood to mean a dimension which is still much larger than the diffraction limit of the light).
On the other hand, a time-dynamic diffuser can reduce the spatial coherence. Here, for example, the dynamic diffuser can be embodied as a moved hologram, a vibrating phase object, a liquid crystal, as an optical phase shifter or as an EO phase modulator. However, a disadvantage of all these processes is that these increase the beam divergence. In particular, a constant phase shift across the whole beam cross section is not expedient in the diffraction far field due to the structures of the speckles which are elongate in the scattering direction.
Some methods are connected with time averaging of speckle fields, for the purposes of which a certain amount of integration time is required. However, since scanning is used as a very quick measurement process with a measurement rate of typically 1 Mpts/s or more, these averaging processes are not employable.
As a further process, a speckle reduction can be obtained by observing/measuring an extended reception aperture in the case where more than one speckle is covered by the reception aperture. The graininess of the light spot at the detector surface becomes finer using an integrated intensity over a plurality of speckles. This intensity integrated at the reception diode has a reduced variation in the intensity.
However, since the size of modern surveying instruments is becoming ever smaller, effective averaging on its own, and hence the speckle reduction, is insufficient.